1. Field of the Invention
The present invention relates to a foreign substance removal unit eliminating foreign substance such as dust, and an optical apparatus including such a unit.
2. Description of the Related Art
In an optical apparatus, such as a digital camera, which performs imaging by converting an object image into an electrical signal, an image sensor receives an imaging light flux and outputs a photoelectric conversion signal. The optical apparatus then converts the output photoelectric conversion signal into image data. In such a case, an optical low-pass filter or an infrared absorption filter is disposed on the object side of the image sensor in the optical apparatus. If foreign substances such as dust adhere to a surface of such a filter, portions where the foreign substances are adhered may be included as black spots in a captured image, so that the visual quality of the image may be reduced.
In particular, in a digital single lens reflex camera with interchangeable lens, mechanical operating units such as a shutter and a quick-return mirror are disposed in the vicinity of the image sensor. Foreign substances, such as dust, may then be generated by the operating units, and may adhere to the surface of the image sensor or the filter. Further, when a user changes the lens, foreign substances may enter the main body of the camera, through an opening in the lens mount and may then become adhered to the surface of image sensor or the filter.
To avoid such phenomena, there is a known method for removing the foreign substances adhered to the surface of an optical filter disposed on the object side of the image sensor, by vibrating the optical filter using piezoelectric elements.
In such a method, it is desirable for the piezoelectric elements to cause the optical filter to vibrate at a resonance frequency so as to produce a large vibration amplitude and thus to efficiently remove the foreign substance. However, the resonance frequency of the optical filter is determined according to its external form, sheet thickness and physicality. The resonance frequency is thus different for each filter due to factors causing variation, such as processing accuracy and a manufacturing process.
Japanese Patent Application Laid-Open No. 2009-17461 addresses this problem through a technique in which a piezoelectric element is arranged in the vicinity of one side of a rectangular optical filter and is located parallel to the side of the optical filter. Further, a detection electrode is disposed on the front side of the piezoelectric element, and a detection ground electrode is disposed on the back side of the piezoelectric element, separately from a driving electrode. The resonance frequency is then searched for by monitoring electrode voltages, while the optical filter is vibrated using the piezoelectric element running through possible resonance frequencies. The dust adhered to the surface of the optical filter is thus removed.
When the optical filter is vibrated, the vibration is reflected at an edge of the optical filter, so that the reflected vibration and the main vibration interfere with each other. Such interference between the vibrations causes unevenness in the size of the vibration amplitude, or causes a vibration phase to be generated in the optical filter, which is different from the phase of the main vibration directly induced by the piezoelectric element. In particular, if the optical filter is rectangular-shaped instead of circular, the effect of the vibration interference further increases.
As a result, if the vibration detection electrodes are disposed at portions which are affected by the vibration interference, unevenness in the size and phase-shifting are generated in the electrode voltages. It thus becomes difficult for the vibration detection electrodes to correctly detect the main vibration. If the vibration detection electrodes falsely detect the main vibration, the piezoelectric element vibrates the optical filter out of the aimed resonance frequency and phase, so that the foreign substance adhered to the optical filter cannot be efficiently removed.